The present invention relates to a machine tool having controlled movement in fixed coordinate directions with limits of travel in each direction. More particularly, the invention relates to an improved apparatus for detecting the limits of travel in each direction.
In practically all multi-axis machine tools which produce relative movement between a tool and a workpiece, it is customary to employ electrical limit stops or switches that remove or terminate energizing power or signals when the tool or a carriage on which the tool is mounted has reached its limit of travel in one coordinate direction. In an XY plotter, for example, there is generally one carriage, denominated the X-carriage, which traverses the work surface of a plotting table in the X-coordinate direction between two limits at opposite ends of the table. A Y-carriage, frequently mounted on the X-carriage to traverse the work surface in the X-direction, moves relative to the X-carriage and work surface in the orthogonal or Y-coordinate direction between two limits. To prevent the carriages from moving too far and thus exceeding the physical and control limits, electrical switches on the table actuated by cams on the carriages are conventionally employed to produce limit signals. The limit signals are employed to terminate motion of the carriages altogether or at least motion of the carriage that has reached the limit of travel. The integrity of the machine is thereby protected.
In the XY plotter described above it is possible that as many as six limit switches would be required for the two axes of motion. Since the X-carriage normally spans the work surface of the plotting table between ways that extend along each side of the table parallel to the direction of motion, a limit stop associated with each end of each way may be required to anticipate slight misalignments or skewing which bring one end of the carriage to its limit before the opposite end. Thus a total of four limit switches may be needed for the X-coordinate axis itself and with the Y-carriage moving back and forth between two other limits, at least two additional limit switches would be needed to secure the system against over-travel in both coordinate directions.
Providing six limit switches at widely spaced points on the plotting table and interconnecting such switches to insure against over-travel entails some expense and increases reliability problems because of the number of components which must be operative.
It is, accordingly, a general object of the present invention to provide an improved limit detecting apparatus that is simpler in construction, less expensive and more reliable.